Prior Art
Exhaust gas sensors for use in the exhaust gas system of motor vehicles to measure the products of combustion are generally supplied with regulated voltage levels. In this manner, the output voltage signal generated by the sensor, which is very small when compared to the supply voltage, can be compared directly in a comparator. The comparator typically has one input from the sensors and a second input from a voltage divider wherein the regulated supply voltage is divided down to a switchpoint or threshold voltage level.
When the exhaust gas sensor is a zirconia sensor, the switchpoint or threshold voltage level is approximately 430 millivolts. The range of the sensor output voltage signal is from a few millivolts to one volt. The zirconia sensor functions as a battery in that the oxidation process of the exhaust gas initiates a chemical reaction and the output signal is a voltage level with a small amount of power.
When the exhaust gas sensor is a titania sensor, the switchpoint or threshold voltage is approximately the same level. The titania sensor, contrary to the zirconia sensor is a variable resistance device. In the typical system configuration, the titania resistor and an ordinary fixed resistor form a voltage divider to a supply voltage. Often, this supply voltage is also used to power and internal heating element on the sensor. At the electrical junction of the fixed resistor and the sensor resistor, the reaction of the titania to the exhaust gas will provide an output voltage signal indicating the composition of the exhaust gas; more particularly the amount of oxygen present in the exhaust. In the typical system, the supply voltage must be regulated, or else errors in determination of air/fuel ratios will result.